For various reasons, including illness, injury or surgery, patients may require replacement or supplementation of their natural renal function in order to remove excess fluid or fluids containing dissolved waste products from their blood. Several procedures known for this purpose are dialysis, hemodialysis, hemofiltration, hemodiafiltration and ultrafiltration; another related procedure is plasmapheresis. The specific procedure employed depends upon the needs of the particular patient. For example, dialysis is used to remove soluble waste and solvent from blood; hemofiltration is used to remove plasma water from blood; hemodiafiltration is used to remove both unwanted solute (soluble waste) and plasma water from blood; ultrafiltration is a species of hemofiltration; and plasmapheresis is used to remove blood plasma by means of a plasmapheresis filter. Because the replacement of renal function may affect nutrition, erythropoiesis, calcium-phosphorus balance and solvent and solute clearance from the patient, it is imperative that there be accurate control of the procedure utilized. The accurate control of the rate of removal of intravascular fluid volume is also important to maintain proper fluid balance in the patient and prevent hypotension.
Various systems have been proposed to monitor and control renal replacement procedures. For example, U.S. Pat. No. 4,132,644 discloses a dialysis system in which the weight of dialyzing liquid in a closed liquid container is indicated by a scale. After the dialyzing liquid flows through the dialyzer, the spent liquid is returned to the same container and the weight is again indicated. Since the container receives the original dialyzing liquid plus ultrafiltrate, the amount of ultrafiltrate removed from the patient is equal to the increase in total weight in the container. This system is not driven by a weight measuring device and does not offer precise control of the amount of liquids used in the procedure.
U.S. Pat. No. 4,204,957 discloses an artificial kidney system which utilizes weight measurement to control the supply of substitute fluid to a patient. In this system, the patient's blood is pumped through a filter and the filtrate from the blood is discharged to a measuring vessel associated with a weighing device. A second measuring vessel containing substitute fluid is associated with a second weighing device and is connected to the purified blood line. By means of a pump, the substitute fluid and the purified blood are pumped back to the patient. The first and second weighing devices are coupled to one another by a measuring system in such a way that a fixed proportion of substitute is supplied to the purified blood stream from the second measuring vessel depending an the weight of the filtrate received in the first measuring vessel. This system does not utilize circulating dialysate fluid in the blood filtration.
U.S. Pat. No. 4,767,399 discloses a system for performing continuous arteriovenous hemofiltration (CAVH). The disclosed system relies upon utilizing a volumetric pump to withdraw a desired amount of fluid from the patient's blood and return a selected amount of fluid volume to the patient.
U.S. Pat. No. 4,923,598 discloses an apparatus for hemodialysis and hemofiltration which comprises an extracorporeal blood circuit including a dialyzer and/or filter arrangement. The system determines fluid withdrawal per unit time and total amount of fluid withdrawn by utilizing flow sensors in conjunction with an evaluating unit located upstream and downstream of the dialyzer or filter arrangement in the blood circuit.
U.S. Pat. No. 4,728,433 discloses a system for regulating ultrafiltration by differential weighing. The system includes a differential weighing receptacle having an inlet chamber and an outlet chamber which allows a fixed amount of fresh dialysate, by weight, to flow through the hemodialyzer. This system operates in a sequence of weighing cycles during which the amount of ultrafiltrate removed from the blood may be calculated. Additionally, the ultrafiltration rate for each weighing cycle may be calculated. This system provides a mechanism for determining and regulating the amount of ultrafiltrate removed from the blood while delivering dialysate to the patient in alternating fill and drain cycles of the inlet and outlet chambers of the differential weighing receptacle.
For certain patients, renal replacement procedures may extend over hours or even days. In general, current systems for monitoring and controlling renal replacement procedures lack the flexibility and accuracy required to perform such procedures on neonates. This is mainly due to the absence of a satisfactory automatic control of the pumps employed. Because of the patient risk involved in using such equipment, health care personnel measure the fluid removed from the patient on an hourly basis. The continuing need to monitor the fluid removed leads to a significant increase in nursing care and thus increases the cost of the therapy. Therefore, there is a need to improve the level of autonomy for the systems such that the procedure is less time consuming for medical personnel, and consequently less costly. However, the enhanced autonomy must not come at the expense of patient safety.
Some conventional renal function replacement/supplementation systems possess an elementary level of supervisory control that simply detects the presence of a fault condition, sounds an alarm, and de-energizes the system pumps to halt the procedure. If the hemofilter clots while the pumps are de-energized, the tubing and hemofilter must be replaced with a concomitant increase in the chance of infection for the patient. Furthermore, the hemofiltration procedure is delayed with a possibly negative impact upon the patient's health.
Due to the time-varying nature of the renal function replacement/supplementation system, the dynamics of fluid pumping may change over time. For example, the characteristics of system components such as tubing, filter, and connectors may vary slowly over time due to aging or as occlusion of the path for fluid flow. As the flow path becomes constricted, the pumping rate of the pump must be altered to compensate for the increased flow resistance. Furthermore, the replacement of a tubing set requires a rapid change adjustment of the pumping rates that may be difficult to initially establish as a relatively constant value due to short-term transient variations. Current systems for monitoring and controlling renal replacement procedures lack the ability to autonomously correct these time-dependent flow rate variations with high accuracy, rapid response, and minimal overshoot or transient variations following correction.
The need exists for a multipurpose renal function replacement/supplementation system which is accurate, reliable, capable of continuous, long-term operation, and which can be used effectively on adult, pediatric and neonatal patients. Further, the need exists for a feedback control system for controlling the multipurpose renal function replacement/supplementation system that accurately regulates the transfer of fluid and monitors the overall behavior of the system to improve patient care and provide greater autonomy.